This invention involves the use of zeolites as ion exchange materials and it particularly relates to preparing granules that are porous and stable in aqueous environments for such applications.
Zeolites have long been known as very efficient ion exchange materials. In order to realize high exchange capacities and rates the zeolites must be quite small crystal size. Such finely divided materials present considerable problems in treating flowing streams, which is the most convenient method of carrying out such processes. In efforts to improve the properties of zeolites in fluid media, beads and granules have been formed. Examples of such efforts are disclosed in U.S. Pat. Nos. 2,973,327 and 3,055,841. The first patent involves the bonding of zeolite crystals with clays to form desired larger bodies of the zeolite. The second patent involves mixing zeolites with a sodium silicate solution, forming the combination into beads by spray drying and exposing the beads to calcium chloride solution to harden them. These materials and others like them have good mechanical properties but are not sufficiently porous and therefore have poor ion exchange rates. U.S. Pat. Nos. 3,764,563; 3,624,003 and 3,382,187 also teach methods directed to improving the properties in fluids as well as other mechanical properties. These methods involve pressure to promote agglomeration, usually in the form of extrusion. While these materials have good mechanical properties, they tend to be less porous than ideal.
A porous granule of small particle size zeolite crystals would have excellent ion exchange properties. European Patent Application No. 0 021 267 and U.S. patent application Ser. No. 455,151, filed Jan. 3, 1983, (U.S. Pat. No. 4,528,276) disclose porous granules of separate particles of zeolite and sodium silicate, but these granules disintegrate, dispersing the zeolite particles throughout aqueous solutions.
It is an object of this invention to provide porous granules of zeolite and silicate that do not disintegrate upon exposure to water.